The invention relates to a hydraulic actuating unit, and particularly to a hydraulic actuating unit for raising a load such as a hospital bed.
Hydraulic actuating units are known which comprise a reservoir for hydraulic fluid, a hydraulic piston cylinder device, the cylinder of which has its ends connected to the reservoir through a supply conduit containing a hydraulic plunger pump with suction and pressure valve means and through a return conduit containing an orifice and a normally closed relief valve. A branch conduit leads to a hydraulic liquid accumulator and connects to the supply conduit at a location between the cylinder and the pressure valve means. A check valve is provided in the supply conduit at a location between the branch conduit connection and the cylinder. A pressure dependent orifice is provided between the check valve and the branch conduit to the accumulator.
These known actuating units are known for use with vertically adjustable hospital beds. They have the advantage of being relatively inexpensive. For raising the bed up to the level of an operating table, for example, the pump (usually of the single plunger type) is manually or foot operated so that hydraulic fluid is intermittently supplied to the hydraulic cylinder. The accumulator operates as a hydraulic shock absorber and minimizes the shocks at the beginning and the end of each pressure stroke of the pump. The supply conduits extending from the branch conduit to the cylinder and the branch conduit are dimensioned relative to each other such that when actuating the unit, the accumulator will be filled up "with priority" through the branch conduit. Lowering of the bed takes place by causing the relief valve to open, usually by means of a foot actuator, so that the hydraulic fluid is permitted to flow from the space under the piston into the reservoir.
When there is an increase in the load while the pump is stationary, for example when a person is sitting down on the bed of the patient, the check valve will prevent the bed from being lowered and hydraulic fluid being expelled toward the accumulator. However, a disadvantage of these known hydraulic actuating units is apparent in the case of a decrease in the load, such as when a patient is being lifted from the bed. When this occurs, the accumulator will supply hydraulic fluid to the cylinder as a result of the decrease in the pressure in the system and thereby will cause an unintended and undesirable raising of the bed. A further disadvantage is that actuation of the relief valve when it is desired to lower the bed will cause the accumulator to completely discharge so that it has to be refilled in the process of the next raising procedure.
The present invention overcomes these disadvantages.